Numerous metallic oxides and sulfides have been proposed as cathode active materials for rechargeable electrochemical cells having a lithium anode. Among the oxides, vanadium oxide V.sub.2 O.sub.5 has been studied to a large extent. It was hoped that electrodes could be obtained having very high energy per unit mass because of the low atomic mass and the high oxidation number (+5) of the vanadium in this material. In addition, V.sub.2 O.sub.5 is substance which is easily obtained and which is easily worked industrially into the form of electrodes.
It is known that during the first discharge of an Li/V.sub.2 O.sub.5 cell, it is possible to insert more than three atoms of lithium per molecule of V.sub.2 O.sub.5 at a plurality of voltage levels on "plateaus" situated respectively at about 3.3 volts, 3.2 volts, 2.3 volts, and below 2.0 volts. However this system is thought to be reversible during a charge-discharge cycle only on the condition that its operation is limited to the two levels above 3 V, which correspond to inserting the first atom of lithium in V.sub.2 O.sub.5. Beyond this limit, it is generally accepted that inserting a greater quantity of lithium gives rise to irreversible modifications in the initial structure of the material, thereby giving rise to a progressive and rapid reduction in capacity during cycling (cf. "Lithium Vanadium Pentoxide Cells" by C. R. Walk in "Lithium Batteries," Academic Press, 1983, p. 265, and "Vanadium Oxides in Electrodes for Rechargeable Lithium Cells" by K. Wiesener et al. Journal of Power Sources 20, 1987, p. 157).
It is also known that the amorphisation of crystalized V.sub.2 O.sub.5 obtained by ultrarapid quenching of molten V.sub.2 O.sub.5 or by forming solid solutions of V.sub.2 O.sub.5 with various different oxides such as P.sub.2 O.sub.5, TeO.sub.2, GeO.sub.2, etc., can serve to increase the reversible domain of lithium intercalation to about 1.5 Li per molecule of V.sub.2 O.sub.5, but in this case the voltage falls off regularly during discharge and the mean voltage over the entire duration of discharge is no more than about 2.6 V (see French patent FR-A-2 573 250, for example).
An object of the invention is to increase the energy per unit mass of rechargeable electrochemical cells including vanadium oxide as a cathode material.